Article pubs.acs.org/crystal
Solution Growth on Concave Surface of 4H-SiC Crystal Hironori Daikoku,*,† Motohisa Kado,† Akinori Seki,† Kazuaki Sato,† Takeshi Bessho,† Kazuhiko Kusunoki,‡ Hiroshi Kaidou,‡ Yutaka Kishida,‡ Koji Moriguchi,‡ and Kazuhito Kamei§ †
Higashifuji Technical Center, Toyota Motor Corporation, 1200 Mishuku, Susono, Shizuoka 410-1193, Japan Advanced Technology Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan § IPS Research Center, Waseda University, 2-7 Hibikino, Wakamatsu, Kitakyushu, Fukuoka 808-0135, Japan ‡
ABSTRACT: A long-term growth of high-quality 4H-SiC single crystals by a topseeded solution growth method using a Si−Cr-based melt was investigated. A new growth technique called “solution growth on concave surface” (SGCS) was developed to help prevent solvent inclusions. The concave shape of the growth surface was achieved by controlling the meniscus height, which enhances the step provision from the periphery to the center. In contrast, under the growth surface, the solution flows from the center to the periphery through convection by inductive heating. The opposite directions of the step flow and solution flow during solution growth create a smooth surface without solvent inclusions. SGCS was used to successfully grow a 1-in. diameter 4H-SiC crystal with a thickness of 30 mm, which is the thickest reported for a solution growth technique, and 1.7-in. diameter high-quality wafers without solvent inclusions were obtained. Schottky barrier diodes were fabricated on 4H-SiC substrates grown by SGCS, which demonstrated breakdown voltages in excess of the 1.2 kV required for hybrid vehicle applications.
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converted to Frank-type stacking faults.11 However, no studies have been reported regarding the electrical properties and device characteristics of solution-grown SiC. One factor obstructing the application of solution-grown SiC to devices is solvent inclusions in the grown crystals, which rule out the use of SiC for device fabrication and evaluation. This study established a new solution growth method enabling the drastic reduction of solvent inclusions to obtain high-quality SiC wafers. In addition, the electrical properties of SBDs fabricated on 4H-SiC wafers grown by this method were investigated.
INTRODUCTION Silicon carbide (SiC) is a wide-bandgap semiconductor material for high-voltage power devices, and it is regarded as a promising material for applications in electrically powered, environmentally friendly vehicles such as hybrid vehicles (HVs), plug-in HVs, full electric vehicles, and fuel cell vehicles due to its high electric field strength, high thermal conductivity, and high electron mobility.1,2 The development of SiC semiconductor devices started in the early 1990s, and the first SiC Schottky barrier diodes (SBDs) were released in 2001. SiC power MOSFETs were then commercialized in 2010.3,4 Mass production of SiC bulk crystals used for the substrates of power devices has been achieved by the physical vapor transport method. The diameter of SiC wafers has been increasing rapidly and recently reached 200 mm. However, this material has not yet been commercially adopted in HVs, because the cost of SiC is much higher than that of conventional Si devices. In addition, currently available SiC crystals contain a variety of crystalline defects that significantly reduce device yield and increase cost. Therefore, low-cost, high-quality wafers are still needed to achieve widespread adoption of SiC power devices. Solution growth has been studied as a method for obtaining a high-quality SiC wafer, because the growth proceeds under conditions close to thermal equilibrium.5 However, one issue with this method is a low growth rate due to the low solubility of carbon in Si solvents.6 Recently, carbon solubility has been increased by adopting solutions containing Ti or Cr,7,8 and a growth rate of up to 2 mm/h has been achieved using a Si−Crbased melt.9 Danno et al.10 reported a dislocation-free bulk crystal with a 0.5-in. diameter produced by the solution growth method, and Yamamoto et al. developed a technique to obtain a high-quality crystal where threading screw dislocations were © XXXX American Chemical Society
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EXPERIMENTAL METHODS
Solution growth was performed in a top-seeded solution growth furnace in helium ambient atmosphere, as shown in Figure 1a. A highpurity graphite crucible was used both as a container for the solution and as a carbon source for the solvent. The graphite crucible was heated by induction coils, and the temperature at the solution surface reached 2000 °C. The temperature gradient from solution surface to 10 mm depth is about 20 °C/cm. The accelerated crucible rotation technique was applied to enhance mixing in the melt and to supply carbon to the surface.12 The rotation rates of seed and crucible are 10 and 5 rpm, respectively. N-type 4H-SiC (000−1) on-axis crystals with a diameter of 0.5−2 in. were used as seed crystals. Silicon−40% chromium-based mixture was melted in the crucible at 150 kPa. The height of the solution was about 30 mm. The inner diameter of crucible was 100 mm. A meniscus forming technique was applied to suppress parasitic reactions such as polycrystalline SiC precipitation around the seed crystal.7 The meniscus height was ranged from 0.5 to 2 mm. During solution growth this height had been Received: August 31, 2015 Revised: January 11, 2016
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DOI: 10.1021/acs.cgd.5b01265 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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spiral center. Step-bunching behavior, characterized as deep grooves between domains, can be observed. Cross-sectional observations in grown layer since 2 mm growth revealed a complicated morphology characterized by black regions and SiC single crystal regions (Table 1b). The black regions were identified as solvent inclusions after the relevant constituent elements were detected by energy-dispersive X-ray spectroscopy (EDX). From these results, solvent inclusions are derived from deep grooves between domains. Step-bunching behavior is generally affected by many growth parameters such as growth temperature, temperature gradient, and solution convection. We speculate that the serious step-bunching leading to groove was caused by differences in local adsorption rates on the growth surface. In other words, the surface roughness increased from atomic level to millimeter level because the raised area can adsorb more Si and C atoms in the solution flow. As a way to suppress different local adsorption rates on the growth surface, we investigated the effect of the growth surface shapes and solution flow on the step-bunching behavior. The shapes adopted were convex and concave. In order to form a convex or concave surface shape, the horizontal temperature distribution of growth surface was controlled by adjusting the meniscus height and dipping depth. The meniscus height is defined as the distance between the crystal surface and solution surface after pulling up the crystal, and the dipping depth is defined as the distance between the crystal surface and solution surface after pushing down the crystal. Figure 2 shows the calculated typical solution flow velocity and the temperature distribution under the seed crystal at dipping, static and pulling condition. The solution flow is directed upward below the center of the seed crystal and moves to the outside along the growth surface. Under the meniscus-forming conditions as shown in Figure 2c, the temperature of the peripheral region is lower than that of the center region due to thermal radiation from the surface of the meniscus bridge. Under the static condition, the temperature of the peripheral region is almost the same as the center region. On the other hand, under the dipping condition, the temperature of the peripheral region is much higher than that of the center region. From these results, the concave or convex surface shape can be controlled by the height of the seed crystal above solution surface. Table 1c−f compares 0.5-in. diameter
Figure 1. Schematic illustration of the growth system of SGCS. adjusted from 0.1 to 0.5 mm/h pulling rate in order to maintain the same value. The pulling rate is not monitored, but experimentally estimated by the rate of declined liquid and the growth rate of crystal. A two-dimensional simulation was performed using CGSim to evaluate the temperature distribution and solution flow.13 After the growth experiment, the surface morphologies of the grown crystals were investigated by optical microscopy using Nomarski interference contrast and atomic force microscopy (AFM). To verify that solution-grown SiC functions as a substrate for power devices, six 1.9 kV-class SBD chips were fabricated on the 4H-SiC substrates produced by solution growth. The off-angle was 4°. The thickness of the drift layer was 10 μm, and the donor concentration was 4 × 1015 cm−3. Ni alloy was used for the anode electrode. After Ni contacts were formed, the post-annealing process was applied at 1000 °C. Copper adhered by Ag paste was used for the cathode electrode. SBDs with 300 μm electrode areas were fabricated. The reverse I−V characteristics were measured up to 1.2 kV at room temperature in insulating oil.
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RESULTS AND DISCUSSION 1. Effect of Surface Shape on Crystal Growth. Table 1a,b shows a plain view and cross-sectional image of a 4 mm thick grown crystal with a flat surface. Cross-sectional observations found solvent inclusions incorporated by serious step-bunching. Table 1a indicates many domains identified to be consisting of a series of macro-step trains emanating from a
Table 1. Comparison of Flat Shape, Concave Shape, and Convex Shape Growth: Top View and Cross Section View with Transmission Mode of Each Growth
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DOI: 10.1021/acs.cgd.5b01265 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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solution flow is similar to the results reported by Sato and Zhu.15,16 In contrast, the convex crystal has bunching caused by the same direction for the step flow and solution flow. 2. Growth Behavior of Concave Shape Growth. The mechanism of stable concave growth was investigated. Figure 4
Figure 2. Calculated temperature and solution flow distribution of meniscus bridge by CGSim at (a) dipping condition, (b) static condition, and (c) pulling condition. The length of arrow indicates the flow rate, and the direction of arrow indicates the flow direction. Figure 4. Surface morphology at concave growth by Nomarski optical microscope at periphery region.
crystals grown with two growth surface shapes. The convex crystal has serious step-bunching and solvent inclusions. In contrast, the concave crystal has a smooth vicinal surface and no solvent inclusions. In other words, it can be established that the suppression of serious step-bunching conduces grown crystal without solvent inclusions. Thus, it can be suggested that the growth technique called “solution growth on concave surface” (SGCS) helps to prevent solvent inclusions. The difference between convex and concave growth can be explained by the relationship between the solution flow and the step flow. Chernov et al.14 and Sato15 used Monte Carlo simulation to show solution flow perpendicular to the steps and in the opposite direction to the step motion prevents bunching. In addition, Zhu reported the suppression of macro-stepbunchings of off-axis crystals under an antiparallel solution flow.16 Figure 3 shows schematic illustrations of concave and convex growth under the different solution flow conditions. The concave crystal has step flow from the outside to the center, and the solution flow is from the center to the outside under the surface. The relationship between the step flow and the
shows the surface morphology of crystals obtained by concave growth. The surface has two regions; a peripheral flat region and a concave region. The peripheral region has flat and large terraces without spirals. This region can be considered as the supply source for steps, and maintains stable crystal growth. The concave region has a consecutive vicinal surface and consists of aligned steps. The step flow is directed to the center region in a concentric pattern, which indicates that the step flow has no surface anisotropy. The relationship between the surface angle and step width was investigated. The surface angle is defined as the angle between the (0001) face and the tangent line of the growth surface, as shown in Figure 5a. Surface morphologies of crystals
Figure 5. (a) Schematic models of surface angle. (b) Surface angle dependence on surface morphologies measured by AFM.
with different surface angles were measured by AFM, and are shown in Figure 5b. The grown surface with a surface angle of 1° had periodical bunching with a width of 2.7 μm and a rootmean-square (RMS) of 6 nm. With a 4° surface angle, the terrace width was 15 μm, and the RMS value was 100 nm. With a 6° surface angle, the terrace width was 60 μm, and the RMS was 420 nm. The step-bunching height and terrace width increased in accordance with the surface angle. We speculate
Figure 3. Schematic illustrations of concave growth and convex growth under upward solution flow condition. (a) The concave surface has been maintained smooth. (b) The convex surface has giant stepbunching caused by the same direction for the step flow and solution flow. C
DOI: 10.1021/acs.cgd.5b01265 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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that the ratio of the terrace width to the step height was adjusted to be constant, so the step-bunching might be accelerated in high-degree surface angle. Under these experimental conditions, it was possible to grow SiC crystals without solvent inclusions at a surface angle below 8°. In contrast, with chemical vapor deposition (CVD), the stepbunching increases as the off-angle of the substrate decreases.17 The values of terrace width and step height in CVD are at least 1 order of magnitude lower than those in SGCS. The stepbunching mechanism with the solution growth method is quite different to that with CVD. Although we speculate that concentration of solute and growth rate have an impact of the behavior of step-bunching, it is quite difficult to describe this mechanism clearly. Figure 6a shows 1.7-in. wafers that were sliced from the grown crystal. The wafers contain no solvent inclusions. Since
Figure 7. (a) Sectional structure of a Schottky barrier diode (SBD). (b) Reverse I−V characteristics of SBDs fabricated on solution growth substrates. Yellow line shows open circuit current.
serious defects that cause leakage current in SBDs, and is the first demonstration that substrates made by the solution growth method can operate as high-voltage devices.
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CONCLUSION 4H-SiC bulk crystals were grown in a Si−Cr-based solvent. To prevent solvent inclusions, the “solution growth on concave surface” (SGCS) method was developed under conditions in which the solution flow is directed from the center to the periphery. It was assumed that a smooth surface is formed by the opposite directions of the step flow and the solution flow. SGCS was used to obtain 1.7-in. wafers without solvent inclusions; 1-in. 4H-SiC crystals were successfully grown with a thickness of 30 mm, and a 2-in. 4H-SiC with a thickness of 10 mm. In addition, it was demonstrated that Schottky barrier diodes on 4H-SiC substrates grown by SGCS have a breakdown voltage of more than 1.2 kV. This is the first proof that substrates produced by solution growth can be used as high-voltage devices.
Figure 6. (a) 1.7-in. 4H-SiC wafers sliced from bulk crystal (not polished). The thickness of this wafer is 0.8 mm. (b) Diagonal views of 1-in. crystal with 30 mm thickness. (c) Diagonal views of 2-in. crystal with 10 mm thickness.
there are no grain boundaries and solvent inclusions in the center region, the step flow from the periphery to the center is presumed to continue and be completed successfully. The polytype of the 1.7-in. wafers was identified as 4H-SiC over all the surface by Raman scattering spectroscopy. The SGCS method successfully provided a 1-in. crystal with a length of 30 mm, and a 2-in. crystal with a length of 10 mm, as shown in Figure 6b,c. 3. Electrical Characteristics. Optical microscopy confirmed that the obtained SGCS SiC crystals had no solvent inclusions. However, the grown crystals include Cr as a impurity at a constant rate. The solvent used in this study includes 40 at.% Cr, resulting in Cr concentration of the grown crystals measured at about 1017 cm−3.18 To clarify the influence of Cr on device properties, the electrical properties of 1.9 kVclass SBDs fabricated on 4H-SiC substrates which were made by solution growth. Figure 7a illustrates the vertical sectional structure of a SBD. Figure 7b shows the reverse I−V waveform of six SBD chips. The measurements found that the breakdown did not occur up to 1.2 kV. The leakage current at 1.2 kV is less than 100 μA, and this value is almost the same as the opencircuit current. This result means that the SiC substrate has no
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the helpful discussions with Prof. Keigo Hoshikawa of Shinshu University. REFERENCES
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DOI: 10.1021/acs.cgd.5b01265 Cryst. Growth Des. XXXX, XXX, XXX−XXX
